Blueish-white, yellowish-white or greyish-white
Does paper really have to be whiter than white? Not necessarily, but if you want to reproduce as many shades in a picture as possible, then the background, i.e. the paper, has to be neutral.
The white we perceive is the light reflected off the paper surface. The greater the difference between the whiteness of the unprinted parts of the paper and the printed sections, the more distinctly we see the colour. The paper therefore has to reflect as much light as possible, while the printing ink must absorb a maximum of the shades that are not to be reproduced.
CIE is the most frequently used method for measuring paper whiteness. The whiteness values resulting from this take into account the fact that the human eye and brain perceive a slightly bluish tinge as whiter than the neutral white of the colour spectrum. The higher the CIE whiteness of a paper, the more it inclines towards bluish-white, while the more yellowish-white papers achieve lower whiteness values.
But don’t let whiteness values alone steer your choice of paper. The most important thing to consider is, after all, the total impression the reader gets from the paper, words and pictures. The most important question to ask is: What impression do I wish to create? A clean, cold feeling or a warm, human one? A blueish-white paper is the best choice for the first alternative. A yellowish-white paper for the second.